The present invention relates to a color image forming apparatus, and, more particularly, to a color image forming apparatus which is capable of preventing deterioration of image quality caused by a variation in the load of an intermediate transfer member.
A typical image forming method for obtaining a color image provides for sequentially repeating the transfer of a plurality of toner images of different primary colors, color by color, from a latent image holding means to an intermediate transfer member, forming a plurality of superimposed color images on the intermediate transfer member, and then transferring the superimposed color images onto a recording medium to form a color image.
As an example of an image forming apparatus using an intermediate transfer member, as described above, there is an apparatus using a driven cylindrical drum as the intermediate transfer member, as disclosed in Japanese Patent Laid-Open No. Hei 1-198773. In the method using an intermediate transfer member, when the size of a recording medium is small, a plurality of images are simultaneously formed on the intermediate transfer member for speeding up image formation. However, in this method, external loads which are intermittently generated after completion of the formation of an image of the previous page of a plurality of sheets affect the image quality of an image of the next page during formation of an image at the same time.
It is necessary to remove residual toner on the intermediate transfer member after secondary transfer of the previous page to the recording medium and before the primary transfer of the next page to the intermediate transfer member. Where a color image is formed on the intermediate transfer member by a plurality of primary transfers, a cleaning apparatus for removing residual toner is disposed apart from the intermediate transfer member for preventing interference relative to the toner image during formation of a color image, and the cleaning apparatus operates in contact therewith in an area of completion of secondary transfer to the recording medium. Because of this, the cleaning operation appears as an intermittent external load relative to the intermediate transfer member.
Further, in the case of using a secondary transfer means requiring pressurization to an intermediate transfer member, such as a transfer roller, and where a color image requiring a plurality of primary transfers is formed, the transfer roller or the like is disposed apart, during formation of color image, from the intermediate transfer member, and operates in contact therewith when secondary transfer takes place, which appears as an intermittent external load relative to the intermediate transfer member.
Where these external loads act intermittently on the intermediate transfer member, a variation in speed caused by the external load is generated in the intermediate transfer member, leading to a variation in speed of a photosensitive member operating as a latent image holding means. When primary transfer or secondary transfer of the next page is carried out at the time of variation in speed, a deviation in position occurs in an image, and a deviation of superposed toner images, called a color deviation, is generated by the positional deviation. This color deviation causes image quality to be deteriorated, making it difficult to obtain a desired image. Therefore, it is necessary to suppress a variation in load and a variation in speed, but the variation of the load itself cannot be eliminated, posing a great technical problem.